Old English dæglic (see day). This form is known from compounds:
twadæglic “happening once in two days,” þreodæglic “happening once in
three days;” the more usual Old English word was dæghwamlic, also
dægehwelc. Cognate with German täglich.

Just an observation: this is not uncommon for forms of words ending in "y". Ready -> readily, gay -> gaily, etc. I guess coyly is a counterexample, but perhaps that is because coily seems as if it might mean "like a coil". :)
–
apsApr 15 '14 at 8:27

@aps In school (English as foreign language) I learned a rule that y becomes i upon adding a suffix, with the exception of vowel + y. Interestingly enough, we never discussed this exception of the exception.
–
YoguApr 15 '14 at 19:04

1

@aps coyly is not a counterexample to a more specific rule: "ay" -> "ai". Are there any examples for "oy" -> "oi"?
–
KazApr 16 '14 at 0:49

@Tyler James Young thanks for the edit that makes the question look quite better :)
–
fedorquiApr 16 '14 at 8:40

3 Answers
3

From the quoted definitions at etymonline, I would suspect that you may be asking the wrong question :)

If I look at the related words in other languages (dag, Tag) for day, it seems the final g has changed into a [j]. The same seems to have happened with (Dutch) leggen -> English lay.

As it is normally pronunciation that defines spelling, and not the other way around, it seems that the i did the job of representing that [j] in daily quite well. However, as a final vowel, the i seems to be uncommon in English, and usually written as a y. (Compare Dutch hooi to English hay).

So where dæglic became daily (the final [k] became [i]), dæg became day. In both cases the g got to be pronounced as [j], but instead of dai, hai or gai, the use of y was preferred to write that sound in a final position.

If I get it correctly, daily comes directly from an original word meaning exactly this. My supposition was that it is a derivative from day, hence the confusion of "why the y became i". That is a nice explanation!
–
fedorquiApr 15 '14 at 22:46

This works as a basic rule of thumb, but with exceptions—one of which, notably, is day/daily (the y is preceded by a consonant). A more accurate way of phrasing the rule: when ‹y› is a vowel /i/, -y => -i-. When it is a vowel /ai/, no change. When it is a glide /j/, there is no rule—you have to learn the words by heart. ‘Day’ and ‘gay’ change to ‘d/gai-’, but ‘gray’ and ‘coy’ do not change.
–
Janus Bahs JacquetApr 15 '14 at 9:28

Consider also play -> playing & played, pay -> paying & paid but say -> saying & said (with change in pronunciation in received pronunciation but not all dialects). If it is acting like a consonant (with -ing) or would double the -i- it has to end up -y-. If it is acting as part of a diphthong (with -ed) it is quite likely not to change from -i- to -y- but there are examples both ways. Historically some -ed words are pronounced with an extra syllable, especially as gerunds/nouns, though this is rare today. I don't know if this is a factor here.
–
David M W PowersApr 15 '14 at 15:19

Also consider the word "slyly", which keeps the "y" despite being preceded by a consonant. Obviously "slily" would have a pronunciation change, which I think is the bigger issue at play here.
–
EricApr 15 '14 at 17:23

Issue with answer: day is not an example of a word ending in a consonant + y.
–
KazApr 16 '14 at 1:12

@Eric, ‘slyly’ is not irregular in the version I gave in a comment above; and there is no reason why ‘slily’ should have to be pronounced differently.
–
Janus Bahs JacquetApr 16 '14 at 6:58

Since I was getting a bit caught up in trying to write out some fairly complex things in comments to @medica’s answer, I am going to write it all out in a full answer here.

Basically, there is a more or less regular variation in some words between final -y and non-final -i- (sometimes -ie) in English orthography. That means that when adding various suffixes to words that end in -y, you have to know whether to change it to an -i- or not, based on a certain set of rules (plus some exceptions—this is English after all). The rules for when to choose what are as follows:

Phonetic quality of -y

The first thing to determine is how the -y is pronounced. There are three possibilities here:

/i/, as in ‘ready’, ‘silly’, etc.

/ai/, as in ‘sly’, ‘shy’, ‘apply’, etc.

/j/, as in ‘day’, ‘whey’, ‘coy’, ‘buy’, ‘dye’, etc.

Regardless of what suffix you’re adding, these different pronunciations affect when the spelling does and does not change.

Phonetic makeup of suffixes

In addition, the pronunciation of the suffix that you add to the end of the word also makes a difference, or rather, the start of the suffix does. Here there are four possible variations:

Combining -y and suffix

When you combine a final -y with a suffix, you have to look at each combination of the seven options mentioned above separately (though some of them can be lumped together, of course).

In the following, I will simply write the -y as its phonetic realisation (/i/, /ai/, or /j/), and as a shorthand, I’ll write the suffixes as simply X (only one consonant), C (consonant-initial), I (starts with /i/), and V (starts with any other vowel). If a rule goes for all types of -y or suffixes, I write ANY. (If two types go together, I just write them together; so CV means ‘suffix that starts with a consonant or a vowel that isn’t /i/’, for instance, and /i ai/ means -y pronounced either as /i/ or as /ai/.)

Exceptions to the main rules are in bold.

(‘Daily’ is in the very last section.)

ANY + I ⟹ -y-

Before a suffix that starts in /i/, -y never changes—it always remains -y-, no matter how it’s pronounced. (Even words that end in -ie change this to -y- here, such as die ⟹ dying, or lie ⟹ lying.)

busy ⟹ busyingsay ⟹ sayingshy ⟹ shyingclay ⟹ clayishdry ⟹ dryish

/i ai/ + X ⟹ -ie-

Before a suffix consisting of only one consonant, -y pronounced /i/ or /ai/ is written -ie-

busy ⟹ busied, busiesdry ⟹ dries, dried

/i/ + CV ⟹ -i-

Before a suffix starting in a consonant or a non-/i/ vowel, -y pronounced /i/ changes to -i-. (This does not happen in the rare instance where the -y pronounced /i/ follows a vowel sound; so gooey ⟹ gooeyness, rather than *gooiness or *gooeiness.)

/ai/ + CV ⟹ -y- (sometimes also -i-)

Before a suffix starting in a consonant or non-/i/ vowel, -y pronounced /ai/ generally remains in monosyllabics, though there are some individual exceptions where a variant spelling with -i- also exists. In polysyllabic words, -i- is the rule.

/j/ + X ⟹ -y- (sometimes -i-)

Before a suffix consisting of only one consonant, -y pronounced /j/ generally remains, except in some irregular verb forms where it changes to -i-. When -y- remains, the past tense suffix -d is written with a preceding e that isn’t pronounced; when -y- changes to -i-, this e is not written.